The objectives of the proposed investigations are to obtain a better understanding of the nature of lesions and processes leading to cell reproductive death following exposure to ionizing radiations, and to study the interrelationships of factors which influence radiosensitivity with an emphasis on their implications for radiotherapy. The hypothesis under examination is that chromosomal aberrations are the principal lesions leading to cell reproductive death. Using cells re-synchronized at each mitosis, we plan to determine the cell survival and loss probability of various aberration types at four successive division attempts following irradiation with gamma rays. In addition, a quantitative comparison would be made of cell killing and aberration production by visible light following different schemes of unifilar or bifilar BUdR incorporation using synchronous cell cultures. The relationship would, in turn, be compared with that obtained using gamma rays. Certain aspects of the BUdR study might also help to clarify the nature of aberrations and chromosome structure. In recent years, much attention has been given to the possible radiotherapeutic advantages of various particle beams, while very few workers have focused their attention toward understanding the successful use of low activity interstitial or intracavitary sources. The proven advantages of the latter may involve dose rate, (1) at low oxygen tensions, and (2) as it relates to the cell life cycle.